Implantable medical devices (IMDs) for cardiac monitoring or for delivering therapy typically include one or more sensors positioned in a patient's blood vessel, heart chamber or other portion of the body. Examples of IMDs include heart monitors, therapy delivery devices, pacemakers, implantable pulse generators (IPGs), pacer-cardio-defibrillators (PCDs), implantable cardio-defibrillators (ICDs), cardiomyo-stimulators, nerve stimulators, gastric stimulators, brain stimulators and drug delivery devices. In a cardiac therapy or monitoring context, such IMDs generally include electrodes for sensing cardiac events of interest and sense amplifiers for recording or filtering sensed events. In many currently available IMDs, sensed events such as P-waves and R-waves are employed to control the delivery of therapy in accordance with an operating algorithm. Selected electrogram (EGM) signal segments and sense event histogram data and the like are typically stored in IMD RAM for transfer to an external programmer by telemetric means at a later time.
Efforts have also been made to develop implantable physiologic signal transducers and sensors for monitoring a physiologic condition other than, or in addition to, an EGM, to thereby control delivery of a therapy, or to filter or store data.
In respect of cardiac monitoring, sensing and recording such additional physiologic signals as blood pressure, blood temperature, pH, blood gas type and blood gas concentration signals has been proposed.
One type of ideal physiologic sensor provides information concerning a patient's exercise level or workload and operates in closed loop fashion. In other words, such an ideal physiologic sensor operates to minimize divergence from an ideal operating point or set of points. Blood oxygen saturation provides a direct indication of the amount oxygen consumed by a patient when exercising. In a rate responsive pacing context, oxygen saturation is generally inversely related to pacing rate. That is, as oxygen saturation decreases due to exercise, pacing rates are correspondingly increased so that divergence from the optimum operating point is minimized. In such a fashion a closed loop system capable of monitoring a physiologic parameter and delivering an appropriate therapy is implemented.
Piezoresistive pressure transducers mounted at or near the distal tips of catheters have been employed in such pressure monitoring applications. U.S. Pat. No. 4,023,562 describes a piezoresistive bridge of four, orthogonally disposed, semiconductor strain gauges formed interiorly on a single crystal silicon diaphragm area of a silicon base. A protective silicon cover is bonded to the base around the periphery of the diaphragm area to form a sealed, evacuated chamber. Deflection of the diaphragm due to ambient pressure changes is detected by the changes in resistance of the strain gauges.
Because the change in resistance is so small, a high current is required to detect the voltage change due to the resistance change. The high current requirements render the piezoresistive bridge unsuitable for long term use with an implanted power source. High gain amplifiers that are subject to drift over time are also required to amplify the resistance-related voltage change.
Other semiconductor sensors employ CMOS IC technology in the fabrication of pressure responsive silicon diaphragm bearing capacitive plates that are spaced from stationary plates. The change in capacitance due to pressure waves acting on the diaphragm is measured, typically through a bridge circuit, as disclosed, for example, in the article "A Design of Capacitive Pressure Transducer" by Ko et al., in IEEE Proc. Symp. Biosensors, 1984, p.32. Again, fabrication for long term implantation and stability is complicated.
In addition, differential capacitive plate, fluid filled pressure transducers employing thin metal or ceramic diaphragms have also been proposed for large scale industrial process control applications as disclosed, for example, in the article "A ceramic differential-pressure transducer" by Graeger et al., Philips Tech. Rev., 43:4:86-93, February 1987. The large scale of such pressure transducers does not lend itself to miniaturization for chronic implantation.
Efforts have been underway for years to develop pressure transducers and sensors for temporary or chronic use in a body organ or vessel, including those relating to the measurement or monitoring of intracranial fluid pressure. Many different designs and operating systems have been proposed and placed into temporary or chronic use with patients.
Patients suffering from head trauma, adult head trauma and infantile hydrocephalus and attendant increased intracranial fluid pressure are often difficult to treat successfully. Among other things, this is because the sensors generally employed to sense intracranial pressure often provide a direct path for infectious agents to enter the brain (leading to dangerous intracranial infections), the actual source or cause of the increased intracranial pressure is poorly understood or not understood at all, or the devices and methods employed to sense intracranial pressure are limited in their capabilities, the locations where they may be positioned, or the durations of time over which they may be used.
Various implementations of systems for sensing physiologic parameters are known in the art. Some examples of such sensors and associated methods of sensing may be found in at least some of the patents, patent applications or publications listed in Table 1 below.
TABLE 1 U.S. Pat. No., U.S. patent Issue/ application Ser. No. Publication/ or Document No. Inventor(s) Filing Date WO 80/01620 Kraska et al. August 7, 1980 H1114 Schweitzer et al. December 1, 1992 B1 4,467,807 Bornzin June 30, 1992 3,669,094 Heyer June 13, 1972 3,746,087 Lavering et al. July 17, 1973 3,847,483 Shaw et al. November 12, 1974 4,114,604 Shaw et al. September 19, 1978 4,202,339 Wirtzfeld et al. May 13, 1980 4,246,908 Inagaki et al. January 27, 1981 4,287,667 Cosman August 4, 1981 4,399,820 Wirtzfeld et al. August 23, 1983 4,407,296 Anderson October 4, 1983 4,421,386 Podgorski December 20, 1983 4,444,498 Heinemann April 24, 1984 4,471,786 Inagaki et al. September 18, 1984 4,467,807 Bornzin August 28, 1984 5,519,401 Ko et al. May 28, 1985 4,523,279 Sperinde et al. June 11, 1985 4,564,022 Rosenfeld January 14, 1986 4,554,977 Fussell November 26, 1985 4,600,013 Landy January 15, 1986 4,621,647 Loveland November 11, 1986 4,623,248 Sperinde November 18, 1986 4,677,985 Bro et al. July 7, 1985 4,651,741 Passafaro March 24, 1987 4,697,593 Evans et al. October 6, 1987 4,727,879 Liess et al. March 1, 1988 4,730,389 Baudino et al. March 15, 1988 4,730,622 Cohen March 15, 1988 4,783,267 Lazorthes et al. April 19, 1988 4,750,495 Moore et al. June 14, 1988 4,791,935 Baudino et al. December 20, 1988 4,796,641 Mills et al. January 10, 1989 4,807,629 Baudino et al. February 28, 1989 4,807,632 Liess et al. February 28, 1989 4,813,421 Baudino et al. March 21, 1989 4,815,469 Cohen et al. March 28, 1989 4,827,933 Koning et al. May 9, 1989 4,858,619 Toth August 22, 1989 4,830,488 Heinze et al. May 16, 1989 4,846,191 Brockway et al. July 5, 1994 4,877,032 Heinze et al. October 31, 1989 4,903,701 Moore et al. February 27, 1990 4,967,755 Pohndorf November 6, 1990 4,971,061 Kageyama et al. November 20, 1990 4,984,567 Kageyama January 15, 1991 4,995,401 Benugin et al. February 26, 1991 5,005,573 Buchanan April 9, 1991 5,040,538 Mortazavi August 20, 1991 5,052,388 Sivula et al. October 1, 1991 5,058,586 Heinze October 22, 1991 5,074,310 Mick December 24, 1991 5,067,960 Grandjean November 26, 1991 5,117,835 Mick June 2, 1992 5,113,862 Mortazavi May 19, 1992 5,117,836 Millar June 2, 1992 5,176,138 Thacker January 5, 1993 5,191,898 Millar March 9, 1993 5,199,428 Obel et al. April 6, 1993 5,267,564 Barcel et al. December 7, 1993 5,275,171 Barcel January 4, 1994 5,291,899 Watanabe et al. March 8, 1994 5,312,454 Roline et al. May 17, 1994 5,324,326 Lubin June 28, 1994 5,325,865 Beckman et al July 5, 1994 5,329,922 Atlee, III July 19, 1994 5,342,406 Thompson August 30, 1994 5,358,519 Grandjean October 25, 1994 5,377,524 Wise et al. January 3, 1995 5,411,532 Mortazavi May 2, 1995 5,438,987 Thacker et al. August 8, 1995 5,490,323 Thacker et al. February 13, 1996 5,535,752 Halperin et al. July 16, 1996 5,564,434 Halperin et al. October 15, 1996 5,556,421 Prutchi et al. September 17, 1996 5,593,430 Renger January 14, 1997 5,601,611 Fayram et al. February 11, 1997 5,617,873 Yost et al. April 8, 1997 5,683,422 Rise November 4, 1997 5,716,377 Rise February 10, 1998 5,743,267 Nikolic et al. April 28, 1998 5,752,976 Duffin et al. May 19, 1998 5,758,652 Nikolic et al. June 2, 1998 5,788,647 Eggers August 4, 1998 5,792,186 Rise et al. August 11, 1998 5,810,735 Halperin et al. May 1, 1997 5,833,709 Rise et al. November 10, 1998 5,873,840 Neff February 23, 1999 09/044,613 Goedeke March 19, 1998 (filing date)
All patents, patent applications and publications listed in Table 1 hereinabove are hereby incorporated by reference herein, each in its respective entirety. As those of ordinary skill in the art will appreciate readily upon reading the Summary of the Invention, the Detailed Description of the Various Embodiments, and the claims set forth below, at least some of the devices and methods disclosed in the patents of listed herein may be modified advantageously in accordance with the teachings of the present invention.